There are a number of applications in which a central control unit must communicate with a number of remote modules over a single communications link. One such application is in the field of computer I/O (input/output) networks for industrial uses. An output module relays commands to machinery, such as on/off commands for a motor. An input module gathers information from a sensor, such as the operating speed or temperature of the motor.
Presently, communication between the controller unit and a large number of remote modules over the single communications link is relatively inefficient. Most systems have a relatively high communications overhead: a relatively large amount of the information transmitted between the controller and the modules relates to addressing and other communications protocol rather than to actual data. Present I/O systems are also inefficient in their use of hardware. Typically, one or more microprocessors are devoted to I/O processing while a separate microprocessor is required for communications.
There are a number of different types of communications protocol for transferring information between a control unit and a number of remote modules over a single communications link. Under one protocol, the modules are individually addressed and information is transmitted to or received from the identified module. Under another protocol, referred to as a time slot mode, modules are activated to receive or transmit information during sequential time slots. However, there is a need for a versatile communications system which can accommodate both commands to specific modules and routine polling of a large number of modules.